Two main categories of folder of the above-mentioned type are known, that is to say, “jaw folders” and “rotary folders”. In these two categories of folder, the retention devices are constituted by pins which engage in the edges of the cut sheets.
In jaw folders, the folding cylinder co-operates with a jaw cylinder in order to fold the sheets. More precisely, the sheets are pushed by the “tucking blades” between the jaws of the jaw cylinder in order to fold the sheets. In rotary folders, the jaw cylinder is replaced by two grooved rollers.
These folders may operate in “straight” mode and in “collect” mode. In straight mode, each sheet is cut, retained on the folding cylinder, then individually folded by means of co-operation of a tucking blade either with the jaw cylinder or with the grooved wheels, depending on the category of the folder. The pins which retain the sheet are simultaneously disengaged from the sheet in order to allow the folded sheet to be discharged in the form of a book. The movement of the pins and the tucking blades is carried out by means of the control cams on which cam followers in the form of rollers which are connected to the tucking blades and to pins travel.
These control cams have, for example, hollow portions in which each roller can be introduced in order to bring about the retraction of the corresponding pins and thus release the sheet, or in order to bring about the projection of the corresponding tucking blade in order to produce a fold in the sheet.
In collect mode, a first cut sheet is retained on the folding cylinder in order to be covered with a second cut sheet. The collect mode therefore allows books to be produced which include a greater number of pages.
In order to prevent the retraction of the pins retaining the first sheet and the projection of the corresponding tucking blade, two covering cams are arranged laterally at the side of the cams for controlling the pins and the tucking blades. These covering cams have projections in the region of the hollow portions of the control cams. The shafts which carry the rollers which are intended to travel on the control cams also carry other rollers which are intended to travel on the covering cams.
In this manner, these other rollers travel on the projections of the covering cams and prevent the rollers associated with the control cams from being introduced in the corresponding hollow portions.
The covering cams therefore neutralise zones of the control cams and prevent, in a controlled manner, the movement of the pins and the tucking blades.
In collect mode, the covering cams rotate about the axis of the folding cylinder at a speed which is generally equal to 2.5 times that of the folding cylinder, when the folding cylinder is provided with five retention devices and five tucking blades. When the folding cylinder includes three retention devices and three tucking blades, the speed of the covering cam is equal to 1.5 times that of the folding cylinder.
In order to move from collect mode to straight mode, the covering cams must be disengaged and immobilised in an angular position in which they do not interfere with the control cams. The operations must be carried out in reverse order to return to collect mode.
This changing of mode therefore requires precise angular positioning, in particular in order to immobilise the covering cam(s) and to ensure that they are reengaged. This is restrictive and further significantly limits the possibilities for automating the changing of the folding mode.